Short-circuit detection devices exist that use current differential signals and real current signals to detect a short-circuit. These devices use a non-trip zone represented in an orthogonal coordinates system with a current differential axis and a real current axis. As soon as a passage outside the zone occurs, a short-circuit is detected. If a current represented by a differential signal and a real component signal remains inside the zone, there is no short-circuit detection.
A state-of-the-art Patent application DE 3,642,136 describes a detection device using current differential signals di/dt and current signals i in an electronic trip device to detect a short-circuit. These signals are applied to a monitoring circuit of a trip zone defined in an orthogonal coordinates system.
In a Patent application EP 838,887 and in U.S. Pat. No. 5,905,615, a circuit breaker is described comprising a short-circuit detector using current differential signals (di(t)/dt)/In and current signals i(t)/In. These signals are applied to a monitoring circuit of a zone without short-circuit defined in an orthogonal coordinates system. In this circuit breaker, the short-circuit detector is associated to a device for monitoring selectivity between circuit breakers.
A Patent application WO 9,903,183 describes a detection method and device using current differential signals di/dt and current signals i to detect a short-circuit in a low voltage mains power system. These signals are applied to a monitoring circuit of a zone without short-circuit defined in the orthogonal coordinates system. Particular zone shapes are defined in this document.
A diagram represented in FIG. 1 shows a circuit breaker 1 comprising main conductors 2 connected to breaking contacts 3 enabling an electrical current Ip to be broken in said conductors 2. A current sensor 4 arranged on a conductor 2 enables a signal Is representative of a current Ip flowing in said conductor to be supplied. The signal Is is supplied to a short-circuit detection device 5. Said device 5 comprises a signal processing circuit 6 and a short-circuit detection processing circuit 7. The circuit 6 has an input to receive the signal Is representative of a current flowing in a main conductor and two outputs to supply signals to the circuit 7. A first output supplies a first signal i representative of the instantaneous value of said current Ip and a second output supplies a second signal di/dt representative of the differential of said current Ip.
Said processing circuit 7 receives the signals i and di/dt and supplies a short-circuit detection signal 8 if an overshoot of a curve corresponding to combined current and current differential characteristics occurs. The signal 8 can control a trip relay 9 of an opening mechanism 10 of the contacts 3.
A FIG. 2 shows a type of combined curve used by devices of the prior art. In this figure, a combined curve 11 is plotted on an orthogonal coordinates system having a first axis 12 bearing values representative of current i and a second axis 13 perpendicular to the first axis 12 and bearing values representatives of current differentials di/dt. This curve 11 corresponding to variable thresholds according to the signals i and di/dt bounds a non-trip zone 14 inside said curve and a trip zone 15 outside said curve.
The methods used in state-of-the-art devices enable simple and efficient short-circuit detection to be achieved, but the speed of decision is not sufficient for very fast detection devices. For certain switchgear, known methods are liable to detect a short-circuit whereas the current is already too high. Such a delay is liable to reduce the performances of a switchgear apparatus such as a circuit breaker.